x86: introduce alloc_vcpu_guest_context()

x86: introduce alloc_vcpu_guest_context()

This is necessary because on x86-64 struct vcpu_guest_context is
larger than PAGE_SIZE, and hence not suitable for a general purpose
runtime allocation. On x86-32, FIX_PAE_HIGHMEM_* fixmap entries are
being re-used, whiule on x86-64 new per-CPU fixmap entries get
introduced.  The implication of using per-CPU fixmaps is that these
allocations have to happen from non-preemptable hypercall context
(which they all do).

Signed-off-by: Jan Beulich <jbeulich@novell.com>